The present invention relates to a method of welding lined metal sheets with an energy beam such for example as a laser beam.
It is in particular applicable to the welding of galvanized metal sheets although it is also applicable to metal sheets lined with any other materials the evaporation temperature of which is lower than the melting temperature of the base material forming the metal sheets or of any multilayer structure exhibiting this characteristic feature.
FIGS. 1 and 2 show the welding with a laser beam 1, which operates in a pulsed or continuous mode, of two metal sheets 2, 3 comprising, on their two opposite sides, respectively, zinc coatings or linings 4, 5 ; 6, 7 and held tightly pressed onto each other by any suitable means in the vicinity of the location of the weld so that both internal linings 5, 6, be comprised between both metal sheets 2, 3. During the displacement of the welding beam 1 in the direction shown by the arrow F1, a weld bead 8 is carried out in order to connect the metal sheets 2, 3 to each other and a so-called capillary passage-way 9 is formed in the zone of molten metal 10. During the welding of the lined metal sheets 2, 3 with the laser beam operating in a continuous mode, owing to the fact that the evaporation temperature of the linings 4-7 is lower than the melting temperature of the base material forming both metal sheets 2, 3, zinc vapour 11 trapped between these two metal sheets pierces the wall of the liquid bath 10 as shown on FIG. 1 and enters the capillary 9 to hit the rear front of the latter as shown on FIG. 2, leading to a capillary which is no longer stable and to an ejection of the molten metal bath 10. The inventors have shown that the piercing of the wall of the liquid bath takes place in one or several tunnels 12 as shown on FIGS. 1 and 2. During the welding of the metal sheets 2, 3 with the laser beam operating in the pulsed mode, the zinc vapour formed during each welding impulse is discharged through the capillary without causing any instability of the liquid bath as shown on FIG. 1. During each pause impulse between two successive welding impulses of the energy beam, the capillary 9 closes as shown on FIG. 3.
It should be pointed out that during the pause impulse, the energy level of the beam is not necessarily zero and that it may still contribute to the welding.
However for the sake of convenience, one should use the expressions xe2x80x9cwelding impulsexe2x80x9d and xe2x80x9cpause impulsexe2x80x9d in the present description to designate the high and low levels of the beam in the pulsed mode.
If the pressure of the zinc vapour is too low, this vapour remains confined in a zone about the liquid bath 10 as shown on FIG. 3. On the contrary if this vapour pressure exceeds a critical pressure, the zinc vapour enters the liquid bath 10 by forming a gas pocket 13 which reaches an equilibrium without any ejection of the liquid bath 10 (FIG. 4), or lifts and fully ejects the liquid bath 10 (FIG. 5). The reference numeral 14 designates a device for feeding gas directed towards the liquid bath 10 and known per se.
It should be noted that during the pause impulse, that portion 10a of the liquid zone 10 which is located towards the place where the metal sheets are not yet welded, may begin to become solidified : that is why it has been hatched on FIGS. 3 to 8.
Many solutions have been proposed for removing the inconveniences due to the uncontrolled evolution of the zinc vapours leading to a weld of bad quality.
One of these known solutions consists in providing, prior to welding, a play or clearance between the metal sheets to be welded to permit the discharge of the zinc vapours. This known solution however has the inconveniences of providing an additional operation of forming bosses for example through pressing on the metal sheets for the formation of the play or clearance and of requiring an appliance adapted for that operation.
Another known solution consists in removing the zinc lining at the location of the weld and in replacing it by a different lining such for example as a nickel alloy. This solution has the inconvenience of causing an excess cost and of being complex owing to the fact of depositing a different lining at some places of the metal sheets.
According to still another known solution, the metal sheets to be welded are arranged vertically and the welding energy beam moves from bottom to top so that the molten material flows away through gravity thereby having the effect of improving the discharge of the zinc vapour. This solution requires a particular mounting in order that the metal sheets be arranged vertically and possibly in the case of complex parts and/or of parts with a great size such as doors of automotive vehicles, to modify the orientation of the part in order that the welding energy beam always moves from bottom to top or upwards.
The object of the present invention is to remove the inconveniences referred to hereinabove of the known solutions by making use of a method of welding lined metal sheets by means of an energy beam which permits to obtain a good weld quality without any adaptation of the geometry of the parts and without using any more or less additional equipment for holding the metal sheets to be welded.
For that pupose, the invention provides a method of welding with an energy beam metal sheets lined or coated with a material having an evaporation temperature lower than the melting temperature of the material of the metal sheets so that the lining material would evaporate during the welding by generating vapour present in the capillary formed by the molten weld material and which is characterized in that it consists in providing an energy beam directed onto the welding zone and adapted so as to promote the discharge of the vapour to the outside in particular through the capillary.
Preferably the energy beam permitting to promote the discharge of the vapour of the material of the lining is the welding energy beam.
According to one embodiment in the case where the energy beam operates in the pulsed mode, the method consists in detecting or providing a modification of the surface of the molten material and in operating the energy beam so that the impulses emitted by the latter may effect a piercing in the molten material when a determined lifting of the surface of this material has been sensed or has been provided in order to discharge the vapour of the lining material present in a pocket of the molten material.
The operation of piercing the surface of the molten material is carried out by decreasing the width of each pause impulse between two consecutive welding impulses of the energy beam or by introducing an additional impulse during the duration of each pause impulse between two consecutive welding impulses of the energy beam.
Preferably the detection of the modification of the surface of the molten material is effected by a source of emission of an incident beam directed towards the molten weld material and a sensor adapted to measure the directional variations of intensity or of shape of the beam reflected by the surface of the molten material, this incident beam advantageously being a laser beam and the sensor advantageously being a photodiode.
The provision of modification of the surface of the molten material is effected by measuring the amount of vapour discharged from the lining material during the welding and by comparing it with a predetermined vapour amount to be discharged of this material, the measurement of the discharged vapour amount being preferably effected by a spectroscopic process.
According to an alternative embodiment still in the case where the energy beam operates in the pulsed mode, the method consists in controlling the energy beam so that the impulses emitted thereby be adapted to effect the piercing of any vapour pocket of the lining material present in the molten weld material during each pause impulse between two consecutive welding impulses of the energy beam in order to discharge the vapour from the material of the lining.
According to still another alternative embodiment according to which the energy beam always operates in a pulsed mode, the method consists in controlling the energy beam so that the impulses emitted by the latter be adapted to put in permanent communication during each pause impulse between two consecutive welding impulses of the energy beam, the capillary and at least one tunnel formed by the vapour pressure of the lining material in the molten interfaces own of two metal sheets to be welded in order to obtain a discharge of the vapour of this material during each pause impulse.
According to either one of the two alternative embodiments mentioned hereinabove, the method consists in decreasing the width of each pause impulse of the energy beam or in introducing an additional impulse during the duration of each pause impulse of the energy beam for piercing the aforesaid vapour pocket or keeping the communication between the aforesaid capillary and tunnel.
The energy beam permitting to promote the discharge of the vapour is arranged so as to provide a capillary having a shape permitting this discharge without any disturbance.
The capillary exhibits a cross section elongated in the direction of displacement of the welding energy beam.
The beam exhibits a substantially elliptical cross section the major axis of which is directed in the direction of the displacement of the welding energy beam.
According to one alternative embodiment, the aforesaid energy beam is inclined in relation to the horizontal for producing the elliptic capillary.
According to another alternative embodiment, one provides one or several cylindrical or aspherical lenses, one or several cylindrical or aspherical mirrors or diffractive optical elements on the path of travel of the energy beam for providing the elliptic capillary.
According to still another alternative embodiment, the aforesaid energy beam oscillates in the direction of the welding energy beam to produce the elliptic capillary.
Still according to another alternative embodiment, one provides at least another energy beam co-operating with the aforesaid first energy beam to produce the elliptic capillary.
Each aforesaid energy beam preferably is a laser beam.